As is known in the art, radio-frequency (RF) power amplifiers (PAs) are important in numerous applications, including RF communications, medical imaging, industrial heating and processing, and dc-dc power conversion among many others. PAs are often required to provide linear amplification, which encompasses the ability to dynamically control an RF output power over a wide range. This becomes particularly challenging when wide-bandwidth control of an output signal is required. It is also often desired to maintain high efficiency across a wide range of output power levels, such that high average efficiency can be achieved for highly modulated output waveforms. Simultaneously achieving both of these requirements—wide-bandwidth linear amplification and high average efficiency—has been a longstanding challenge.
As is also known, one concept that has been explored for achieving both linear operation and high efficiency is referred to as outphasing. This technique is also sometimes referred to as “Linear Amplification with Nonlinear Components” or LINC. As shown in FIG. 1, in conventional outphasing, a time-varying input signal Sin(t) is decomposed into two constant-amplitude signals S1(t), S2(t) which can be summed to provide a desired output signal Sout(t). A variable envelope output signal is created as the sum of two constant-envelope signals S1(t), S2(t) by outphasing of the two constant envelope signals. Because the two signals S1(t), S2(t) are of constant amplitude, they can be synthesized with highly-efficient PAs including partially- and fully-switched-mode designs such as classes D, E, F, E/F and current-mode D, Inverse F, φ, etc. These amplifiers can be made highly efficient in part because they needn't have the capability to provide linear output control. Combining the two constant-amplitude outputs S1(t), S2(t) in a power combining network enables the net output amplitude to be controlled via the relative phase of the two constituent components S1(t), S2(t).
One important consideration with outphasing is how the power combining is done, particularly because many high-efficiency power amplifiers are highly sensitive to load impedance, and their performance and efficiency can heavily degrade due to interactions between the power amplifiers. As shown in FIG. 2, one conventional approach is to combine the constant-amplitude signals S1(t), S2(t) using an isolating combiner. An isolating combiner provides a constant (resistive) loading impedance to each PA independent of the outphasing angle, eliminating any interactions. A consequence of this, however, is that each PA operates at a constant output power level. Power that is not delivered to the output must instead be delivered elsewhere, usually to an “isolation” resistor R which dissipates power in the form of heat. Thus, a portion of the total constant output power from the PAs is delivered to the output (at the sum port of the combiner); the remainder is delivered to the difference port and is lost as heat in the isolation resistor. This leads to a rapid degradation of efficiency as output power is decreased, diminishing the attractiveness of this approach. This problem can be partially offset by recovering power not delivered to the output through a rectifier. Thus, in some implementations, power not delivered to the output is instead recovered back to the dc supply via a rectifier.
Referring now to FIG. 3, a different conventional approach is to use a lossless combiner, such as a Chireix combiner or related methods. (By “lossless” we mean a combiner including only reactive components or energy storage components such that ideally there would be no loss, while recognizing that all real components have some degree of loss. We also refer to a “lossless” combiner as a reactive combiner.) Benefits of the Chireix combining technique, which is non-isolating, include the fact that the combiner is ideally lossless, and that the real components of the effective load admittances seen by the individual power amplifiers vary with outphasing (and power delivery) such that power amplifier conduction losses can be reduced as output power reduces. However, the reactive portions of the effective load admittances (Yin,1, Tin,2) are only zero at two outphasing angles, and become large outside of a limited power range (see the example plots in FIG. 3). This limits efficiency, due both to loss associated with added reactive currents and to degradation of power amplifier performance with (variable) reactive loading. Stated differently, the reactive impedances +jXc, −jXc of the combiner compensate for the effective reactive loading on the PAs due to interactions between them. However, because the effective reactive loading due to PA interactions depends upon operating point (outphasing angle), compensation is imperfect over most of the operating range. This can thus lead to loss of efficiency and PA degradation when operating over wide ranges.
Accordingly, the above-described challenges with power combining are among the principal reasons that outphasing is not a more dominant architecture in RF applications.
It would, therefore, be desirable to provide a power combining and outphasing modulation system for use in RF applications that overcomes the loss and reactive loading problems of previous outphasing approaches by providing ideally lossless power combining, along with substantially resistive loading of the individual power amplifiers over a very wide output power range, enabling high average efficiency to be achieved even for large peak-to-average power ratios (PAPR).